Dihydroorotate dehydrogenase of Toxoplasma gondii - kinetic characterization and intracellular localization

Abstract

The pyrimidine biosynthesis patway in the protozoan pathogen Toxoplosma gondii is essenial for parasite growth during infection. To investigate the properties of Dihydroorotate dehydrogenase (TgDHOD. TgDHOD exhibited a specific activity of 83.8 U/mg, a kcat of 89.2 sec-1 ± 1.5. a Km = 60.3 ±0.002 æM for L-dihydroorotate, and a Km = 28.9 æM ± 1.8 for decylubiquinone (QD). Quinones lacking or having short isoprenoid side chains yielded lower kcats and higher Kms than QD. As expected, fumarate was a poor electron acceptor for this family 2 DHOD. The determined for Redoxal and A77-1726 were 253.3 æM ± 13.3 and 91.2 æM ±2.2, respectively. The enzyme was not significantly affected by brequinar or TTFA, known inhibitors of human DHOD, or by atovaquone. TgDHOD exhibits a 157 ? residue N-terminal extension, consistent with potential organellar targeting, but bioinformatic analysis failed to reveal a consensus subcellular destination, Preliminary inmunolocalization studies of TgDHOD performed in intra- and extracellular parasites treated with the polyclonal antibodies raised against purified recombinant TgDHOD exhibited fluorescence that appeared to colocalize with the apicoplast in fluorescence was observed in -10% of mitochondria stained with Mitotracker Red in extracellular parasites. These findings suggest that TgDHOD is associated with both the apicoplast and mitochondrion, making it a member of a growing list of a metabolic enzymes that are dual targeted in T. gondii